1. Field of the Invention
The present invention relates to the control of image density in an image formation apparatus.
2. Related Background Art
As a conventional example, the structure of a multi-color image formation apparatus as well as its operation will be described with reference to FIG. 1.
In FIG. 1, latent images formed every each of colors on an image support body (photosensitive drum) 100 by an optical unit 101 are developed and visualized using each of color toners of Y (yellow), M (magenta), C (cyan) and K (black) respectively supplied from color developing units of Dy, Dm, Dc and Dk. Then the developed and visualized images are plural times transferred on an external surface of a transfer belt 102 so as to form a multi-color image. In this condition, the toner is transferred to a surface of the transfer belt 102 by applying high voltage on the transfer belt 102. Then a recording sheet (paper) 105 fed from a sheet feed unit 103 or a sheet feed toner 104 is conveyed through a sheet conveying path and the multi-color image is re-transferred from the transfer belt 102.
Thereafter, the recording sheet 105 is conveyed by a conveying roller 106 and is fixed by a fixing unit 107 to be discharged to a discharge tray 108 or a discharge unit 109. The each of color developing units, which has a rotation spindle at its both edges, capable of being rotated around the spindle is held in a developing mechanical unit 110 and is rotated to be selected. Numeral 111 denotes a cleaning unit for cleaning the toner on the transfer belt 102. Numeral 112 denotes a discharged toner collection unit for collecting discharged toners from the image support body 100. Numeral 113 denotes a density sensor for measuring density of a toner image formed on the photosensitive body 100.
In the above-described structure, as shown in FIG. 2, a beam is radiated from a light emitting element 1 structured within the density sensor 113, which is disposed in the vertical direction to a surface of the image support body 100, to a toner image 20 (called as patch hereinafter) formed on the image support body 100. Then a reflected light is detected by a light reception element 2 to realize such a structure as stabilizing image density by correcting a difference between the detected result and a predetermined detection level as a change quantity corresponding to a developing bias.
An example of the content concerning a density control will be described. The density control can be categorized into a developing bias control for obtaining a developing bias value treated as a maximum value of toner density and a halftone density control for controlling halftone density by varying image data upon fixing the developing bias value defined by the developing bias control. Hereinafter, the developing bias control will be described.
FIG. 3 is a block diagram of the structure concerning the developing bias control. In FIG. 3, numeral 3 denotes an A/D converter which converts an analog detection signal transferred from the density sensor (light reception element) 113 into a digital signal. Numeral 4 denotes a data comparison means which judges whether or not an output value of the density sensor for a surface ground of the image support body reaches a predetermined value. Numeral 5 denotes an LED light quantity setting unit which varies light quantity so as to secure an output range in case of measuring several kinds of patches.
Numeral 6 denotes a data storage means which interpolates detected data. Numeral 7 denotes a density calculation unit which changes a sensor output value of the measured patch in terms of a density value. Numeral 8 denotes a density/developing bias comparison unit which determines the density value changed by the density calculation unit 7 and the developing bias value corresponding to the density value.
Numeral 9 denotes a developing bias control unit which gives a command to a high-voltage output unit 10 so as to output the determined developing bias value. The high-voltage output unit 10 applies an output designated from the developing bias control unit 9 to the developing unit. Numeral 50 denotes a CPU within a DC controller (not shown) provided in an image formation apparatus main body. Depending on the structure, each of the blocks 3, 4, 5, 6, 7, 8 and 9 provided in the CPU 50 may be provided in the DC controller (not shown) or the density sensor 113.
FIG. 4 is an operational flow chart of the developing bias control. In FIG. 4, the ground of the image support body surface is measured in a step S101. A flow advances to a step S102, where if a read value of the ground of the image support body surface is lower than a predetermined density value, the flow advances to a step S104. If the read value of the ground of the image support body surface is higher than the predetermined density value because of dirt or an inferior change in time of the density sensor 113, a density sensor output is corrected in a step S103.
The density sensor output is corrected by each block of the A/D converter 3, data comparison means 4, the LED light quantity setting unit 5 and the data storage means 6 shown in FIG. 3. In the step S104, density of the ground surface on a position, where the patch on the image support body is to be printed, is measured. Then density of the patch is measured in a step S105. In this case, the density of the patch for the ground can be measured as contrast by measuring the density of the ground in the step S104. The flow advances to a step S107 after converting the read value of the density sensor 113 into the density value in a step S106. A method for determining an optimum developing bias value performed in the step S107 will be described hereinafter.
FIG. 5 shows examples of measured patches. A patch 20-a is in the lowest density and a patch 20-e is in the highest density. The density is schematically expressed by hatching lines. The number of patches is not limited to these examples but may be varied depending on a diameter of the image support body or time spend in controlling the density. FIG. 6 shows the relationship between the density value being the measured result of patches 20-a, 20-b, 20-c and 20-d shown in FIG. 5 by the density sensor 113 and the developing bias value when the patches are formed.
In FIG. 6, in a case where a target density value want to be obtained among the measured density of five patches exits on somewhere between two points, the optimum developing bias value for the target density can be obtained by performing a linear interpolation for the two points.
FIG. 7 indicates a case that all patches which are measured can not reach the target density. In this case, the linear interpolation is performed for the two patches of which density is closer to the target density so as to estimate the optimum developing bias value for the target density.
FIG. 8 indicates a case that all patches which are measured can not reach the target density and the density value reaches peak between the patches 20-b and 20-d. In this case, the developing bias value of the patch of which density is closest to the target density (patch reaches a peak of the density) is treated as the optimum developing bias value.
The halftone density control is performed after determining the optimum developing bias indicating a maximum density by the developing bias control. Also, in case of the halftone density control, a plurality of patches are printed on the image support body to measure the patches density by the density sensor 113 similar to the case of the developing bias control. The relationship between image data of the halftone density control patch and the density value is shown in FIG. 9. On an image data/density characteristic curve shown in FIG. 9, since a raise of the density is remarkable in the vicinity of center position of the image data, a halftone correction curve is obtained by calculation so as to perform such a process as correcting the characteristic curve in linear as shown in FIG. 10. Consequently, reproductive precision of halftone density, which is largely changed in color reproductivity of an image, can be improved.
However, conventionally, in measuring of the patch density when toner image density is controlled, there occurs such an inconvenient situation as an output voltage when a beam is radiated to the image support body (assumed as reference voltage V0) becomes more sensitive for the light due to a material characteristic of the image support body of which surface is scraped off because of a long period use of a color image formation apparatus. Also, there occurs such an inconvenient situation as resulted in deteriorating light sensitivity because of dirt of the density sensor due to dirt of toners in the image formation apparatus.